Micro-resonators fabricated with microelectromechanical system (MEMS) technologies are used for multiple applications, including low-power, low-phase noise, and high stability oscillators for clocking and timing. The performance of MEMS resonators is usually sensitive to the change of environment such as stress, temperature, acceleration etc. In these designs, challenging aspects include connecting the MEMS resonators to the circuit, and packaging. Multiple approaches have been used, such as: wire-bonding, flip-chip MEMS, CMOS-MEMS, etc.
An acoustic wave resonator (AR) or a thin-film bulk acoustic wave (BAW) resonator is an example MEMS device that includes a piezoelectric material, which is sandwiched between two electrodes and acoustically isolated from the surrounding medium. When MEMS resonators use piezoelectric films with thicknesses ranging from several micrometers down to tenth of micrometers, they resonate in a frequency range of roughly 100 MHz to 10 GHz. Aluminum nitride (AIN) and zinc oxide (ZnO) are two common piezoelectric materials used to operate at this frequency range.
Several types of MEMS resonator based oscillators are known, such as: (a) Paidimarri, Arun, Nathan Ickes, and Anantha P. Chandrakasan, “A 0.68V 0.68 mW 2.4 GHz PLL for Ultra-Low Power RF Systems,” 2015 IEEE Radio Frequency Integrated Circuits Symposium (RFIC) (May 2015); and (b) A. Nelson, J. Hu, J. Kaitila, R. Ruby and B. Otis, “A 22 μW, 2.0 GHz FBAR oscillator,” 2011 IEEE Radio Frequency Integrated Circuits Symposium, Baltimore, Md., 2011, pp. 1-4. A high-Q MEMS-based oscillator may have moderate power consumption, low phase noise, and high accuracy/stability.